Security printing methods have used invisible inks as a means of verifying data or providing additional information relating to documents. Invisible inks have been known for quite some time. In the past, invisible ink has typically been used to write secret messages and/or codes, and is configured to become visible upon treatment with heat and/or chemicals. Invisible inks are often still used as novelty items, children's toys, and for encoded data, where the invisible ink can be developed to become visible by employing appropriate chemistry.
As ink-jet technology has become widespread and readily available, various security techniques including the use of invisible inks have been utilized in conjunction with ink-jet printing. Such invisible inks have included the use of specialized dyes or pigments. For example, such dyes include UV or near infra red dyes. Additionally, metallic or ceramic particles having unique optical, electrical, or magnetic properties have been employed in security printing mechanisms configured for ink-jet printing. However, security measures employing such sophisticated techniques can be quite costly employing expensive chemicals. Accordingly, investigations continue into developing cost-effective security printing methods that can be employed in the ink-jet arts.